In the hermetic enclosure of silicon semiconductors, the semiconductors are connected in many instances to ceramic bases which can also be metallized, e.g. silver-plated or gold-plated.
Recently, silver-filled glass solder pastes have been used for this purpose. These pastes consist of finely distributed silver powder and glass powder, mixed with a solvent and a thermally decomposable resin. These pastes are applied onto the ceramic bases in a thin layer, the semiconductor is put on, and the structure is dried at approximately 100.degree. C. and heated to temperatures above 400.degree. C. During this time solvent and resin escape, the glass softens and produces, together with the silver powder, a firmly adhering bond between the base and the semiconductor.
Silver-filled glass solders are used today especially in the manufacture of CERDIP components (CERDIP=ceramic dual inline package) and have largely replaced the previously-conventional connection technology which employed a gold/silicon solder (AuSi 2-solder).
However, the trend in electronic manufacturing is toward larger and larger, more and more complex and more and more highly integrated semiconductor components, which are very temperature-sensitive and do not withstand the burning temperatures of 400.degree.-450.degree. C. which were necessary in the bonding process which employed silver-filled glass solders. Diffusion processes occurring on the semiconductor are accelerated at these temperatures, and these processes can destroy the fine, metallic conducting tracks.
U.S. Pat. No. 4,401,757 discloses a metallizing paste for fastening silicon semiconductors onto ceramic bases which consists of 75-85% by weight of a mixture of 20 to 95% by weight finely distributed silver, 5-80% by weight of a low-melting glass powder and 15 to 25% by weight of an organic solvent. The glass used has a softening point between 325.degree. and 435.degree. C. and consists essentially of 95 to 96% by weight lead oxide, 0.25 to 2.5% by weight silicon dioxide, remainder boron oxide. It thus comes from the class of lead borosilicates. The bonding between the silicon semiconductor and the ceramic base takes place at burning temperatures between 425.degree. C. and 525.degree. C., especially at 430.degree. C. Holding times of 5 to 10 minutes are necessary for achieving a good adhesive strength with this material.
U.S. Pat. No. 4,636,254 describes a silver glass paste which contains a lead borate frit and, selectively, silver oxide powder, in addition to a special silver powder mixture. In this case, the connection of the semiconductor to the ceramics also takes place preferably at 430.degree. C. The silver oxide powder is added directly to the paste and serves to improve the burning of the organic paste constituents.
The solder pastes disclosed in U.S. Pat. Nos. 4,401,767 and 4,636,254 are disadvantageous because of the high working temperatures of approximately 430.degree. C. as well as the long heating times of 5 to 10 minutes which are necessary for processing. These high temperatures and long heating times damage both the semiconductor and the housing materials. They can cause thermal stresses and fissures in the semiconductor material as well as initiate diffusion processes in the metallic conducting tracks which result in the destruction of the component. Such diffusion processes can occur especially in the case of semiconductors with a very fine line structure ("sub-micron-technology"), so that protective conditions for burning in are absolutely necessary. The high working temperatures of approximately 430.degree. C. cause considerable diffusion of nickel from the nickel/gold layers of the ceramics in semiconductor housings, especially in the case of so-called multilayer housings with a high connection number (sidebraze packages, leadless ceramic chip carriers and pin grid arrays) . This results in increased defects in the subsequent hermetic sealing by means of gold/tin solder.
U.S. Pat. No. 4,699,888 describes a silver glass paste suitable for lower burning temperatures around 380.degree. C. This solder paste contains a special lead borosilicate glass with additives of Cu.sub.2 and fluoride in addition to silver powder and binding agent. This does achieve a lowering of the glass melting point and the burning-in temperature of the solder paste. However, this solder paste is disadvantageous because of its fluoride content; moreover, the burning temperature of 380.degree. C. is still too high for use with modern, highly integrated semiconductors.